From the look of the forecast, Erin Furey-Patterson of Weston figured that Wednesday would be a snow day for her two young children. With Boston bracing for up to 10 inches of snow and bitter cold, state employees were sent home early Tuesday, and Logan Airport canceled flights in droves. By midafternoon, classes city schools had already been canceled for the next day.

But Furey-Patterson woke up to find that the powerful storm, which dropped more than a foot of snow across a swath of towns south of Boston, had dealt only a glancing blow to others. In Weston, only a few inches had fallen, and schools were open after all.

Breaking the news to her second-grader Charlotte, who had laid out her snow pants and mittens the night before in anticipation, was not easy. Worse still, Charlotte’s younger sister, Claire, who goes to kindergarten in Brookline, had the day off.

Defying predictions, the hit-or-miss storm walloped towns south of Boston, while merely dusting those to the north. Boston got a mere 4 inches, while Norwell, just 25 miles to the south, got 18.

Yet schools were closed in many communities that escaped the storm’s wrath, including Newton, Cambridge, and Somerville. Like Boston, many school systems announced the closings before the storm arrived, part of a growing trend aimed at giving parents more time to make other arrangements for their children.

“They worry about getting the word out as quickly as possible, so people can plan,” said Thomas Scott, who directs the Massachusetts Association of School Superintendents. “Generally speaking, they are going to take a conservative approach, and they pay a lot of attention when they hear forecasters predicting really serious storms.”

“People are going to say, ‘You jumped prematurely,’ ” Walsh said. “It’s cold. With all the salt, it is wet. I’d rather be safe than sorry.”

But while making early calls has clear advantages, Wednesday’s cancellations illustrate the obvious drawbacks, a snow day when there is barely any snow.

“There’s an advantage for families to arrange child care,” said Cheryl Maloney, superintendent of the Weston public schools. “The problem is, it’s much easier to call off school when you don’t need to.”

Maloney said that on Tuesday night, the storm’s impact on the suburbs west of Boston was unclear, so she waited until Wednesday morning to decide. After waking at 4 a.m., she spoke with the police and the Public Works Department, who made it clear that the roads were fine.

In Newton, superintendent David Fleishman said he decided to close schools about 8:30 Tuesday evening in light of the worsening forecast.

“We were all in that circle that was supposed to get hit,” he said. “Most people went with the percentages.”

‘There’s an advantage for families to arrange child care.’

CHERYL MALONEY, superintendent of the Weston public schools

When Fleishman awoke to little more than a dusting, he was taken aback.

“This was a real surprise,” he said. “I do feel for families. I am very sensitive to child-care issues and the inconvenience of school being closed.”

Bill Simpson, a meteorologist at the National Weather Service in Taunton, said even slight changes in conditions can substantially change a forecast. In this case, “a shift of 30 miles along the storm track made all the difference,” he said.

Making an early call is meant to avoid forcing parents to scramble for child care at the last minute, but school closings, no matter what time they are announced, can be a hardship.

“I think today was a reminder that early calls are not always best,” Fleishman said.

“We live in New England: We should be able to go to school when there’s a little snow,” she said. “It seems they are making decisions earlier and earlier now, and I wish they would hold off a bit.”

A marketing director, McEachern worked from home Wednesday to look after her two children, 12 and 14.

Calling unnecessary snow days also pushes the end of the school year further into June, parents complained. State regulations require that students attend school for 180 days, and school systems usually allow for only five snow days on the calendar.

But while parents tend to criticize school officials for hasty cancellations, a new study by a Harvard University researcher suggests that administrators should call off classes more often when the weather is iffy.

Joshua S. Goodman, an assistant professor at the Harvard’s John F. Kennedy School of Government, said that while snow days do not appear to hurt academic progress, keeping schools open during storms may be detrimental for the students who may not be able to make it in and their classmates.

When the whole day is canceled, teachers simply resume their lessons for everyone when students return. But when school stays open during inclement weather, the students who miss class are likely to fall behind, potentially slowing down the whole class.

“What’s really hard to deal with is when schools open and five of 25 students don’t show up,” said Goodman, a former teacher who studied the issue at the urging of state education officials, curious about the impact of snow days on standardized test scores. “Student absences force teachers to expend time getting students on the same page.”

Wesley Lowery and Andrew Ryan of the Globe staff contributed to this report. Peter Schworm can be reached at schworm@globe.com.